1. Field of the Invention
The present invention relates to a display device using a liquid crystal layer as a display medium.
2. Description of the Related Art
FIG. 35 shows an example of the configuration of an active matrix substrate serving as part of a liquid crystal display device, using thin film transistors (hereinafter, referred to as TFT) as switching elements. A plurality of TFTs 402 and a plurality of storage capacitors 401 connected to the respective TFTs 402 are formed in a matrix on the active matrix substrate. Each gate line 403 is connected to each gate electrode of the TFT 402. The TFT 402 is driven by a signal input to an input terminal 403a so as to be applied to a gate electrode. A source line 405 is connected to a source electrode of the TFT 402, to which a video signal input from an input terminal 405a is applied. The pixel electrode and one terminal of the storage capacitor 401 are connected to a drain electrode of the TFT 402. The other terminal of the storage capacitor 401 is connected to a common line 404 functioning as a storage capacitor counter electrode, and thus is connected to a counter electrode formed on a counter substrate when the active matrix substrate and the counter substrate are attached to each other. The active matrix substrate and the counter substrate are attached to each other so as to interpose a liquid crystal layer therebetween, thereby completing a liquid crystal display device.
In order to realize a color display in this liquid crystal display device, the configuration in which a color filter consisting of filter portions of three colors (e.g., RGB) is formed on a counter substrate is most generally adopted. In this case, a black mask is generally formed on a counter substrate so as to prevent the mixture of colors and the light leakage. The black mask prevents light leakage from a voltage-unapplied portion of the liquid crystal layer of the pixel when it is provided in the boundary region between the pixels and prevents light leakage from the peripheral region when it is provided in the peripheral region surrounding a display region. FIG. 36 shows the active matrix substrate 410a having the configuration shown in FIG. 35 which is attached to a counter substrate 410b. For simplicity, FIG. 36 does not show the storage capacitors 401, the TFTs 402 and the storage capacitor counter electrode 404 on the active matrix substrate 410a. Moreover, a black mask provided in the boundary region between pixels within a display region 450 on the counter substrate 410b is omitted in FIG. 36, and only a black mask 430 provided in the peripheral region surrounding the display region 450 is shown. The black mask 430 should be formed in the peripheral region so as to conduct a high quality display. Although not shown in FIG. 36, a suitable margin is provided between the black mask 430 in the peripheral region and the edge of the counter substrate 410b.
As described above, in the conventional color liquid crystal display device, the black mask 430 is formed along with a filter pattern (not shown) of RGB on the counter substrate 410b. The elimination of the step for forming the black mask 430 would be effective in reduction of the fabrication cost of a liquid crystal display device. Therefore, it is necessary to develop a liquid crystal display device which does not require the black mask 430 to be formed on the counter substrate. In order to achieve such a liquid crystal display device, however, light leakage from the peripheral region surrounding the display region needs to be prevented even without using a black mask formed on the color filter substrate.
Moreover, the black mask 430 made of a resin does not have sufficiently high optical density value, and has inferior reliability to that of a black mask made of a metal. However, the black mask 430 made of a light shielding metal has the following problems.
The active matrix substrate 410a and the counter substrate 410b are attached to each other while a sealing resin is applied to the peripheral region of any one of the substrates. In the peripheral region, an electric field is generated between the black mask 430 provided on the counter substrate 410b and the gate line 403 and the source line 405. Therefore, part of the black mask 430, which is positioned outside the region to which a sealing resin is applied corrodes due to electrochemical reaction with atmospheric moisture contained.
On the other hand, the black mask provided in the peripheral region outside the display region 450 should have a relatively large width. If the black mask does not have a sufficiently large width, the assembly accuracy needs to be increased, resulting in increased assembly cost. A width of the black mask should normally be about 2.5 mm in the peripheral region. As a result, the reduction of the peripheral region for reducing the weight and size of a liquid crystal display device is limited due to the corrosion described above.